It wasn't a hill to die on, it was a private donation from his private bank account as a private person. The funding page got leaked and people saw that he donated. And it wasn't removing the right for gays to marry, it was to keep the proposal from becoming a law. You might not realize that some people hold very strong religious beliefs about marriage, something the vast majority of Americans historically had, and for this reason gays previously generally only had something called a civil union prior to 2015.